1. Field of the Invention
The present invention relates to fiber optics, and in particular the invention is directed to a multi-channel tunable filter.
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2. Background Art
Digital and analog information is often communicated using optical fibers. In some schemes, many signals, each with its own optical wavelength, are communicated on the same optical fiber. At some point, it is necessary to add or to extract a signal (i.e. a particular optical wavelength) from the optical fiber and this is accomplished with an optical add/drop filter. Various type of optical filters have been developed for use in telecommunications. Fixed wavelength optical filters are the most commonly used in today""s networks to filter optical channels off a multiplexed stream of wavelengths. A problem with fixed wavelength filters is that they are limited to a single fixed optical wavelength. The increasing rate at which information is transferred makes the network increasingly difficult to manage. Network manageability can be simplified by selectively routing information at the wavelength level. This can be understood by a review of optical signal transmission schemes.
With the rapid emergence of the Internet, there is a great need to increase the volume of data that can be transmitted across a network of computing devices (commonly termed bandwidth). Initially, optical fiber networks carried only a single signal at a single wavelength. The bandwidth of optical fibers was increased by using a scheme known as wavelength division multiplexing (WDM).
The concept of WDM is to launch and retrieve multiple data channels in and out, respectively, of an optical fiber. Prior to the use of WDM, most optical fibers were used to unidirectionally carry only a single data channel at one wavelength. WDM divides a network""s bandwidth into channels, with each channel assigned a particular channel wavelength. This allows multiple signals (each at a different wavelength) to be carried on the same transmission medium. For example, multiple optical channels can be used with fiber optic cable to transmit multiple signals on the same cable. The gain in the network bandwidth is given by the aggregation of multiple single channel bandwidth.
In most situations, the channels are merged (multiplexed) at a transmitting end and transmitted to a receiving end where they are separated (demultiplexed) into individual signals. In the existing systems, the transmitting and receiving ends must be tuned to the same wavelengths to be able to communicate. That is, the transmitting and receiving ends use a device such as an add/drop multiplexer to transmit/receive a fixed signal channel. In the case of fiber optic cable, an optical add/drop multiplexer can be used at nodes or at the receiving ends to generate a fixed wavelength (e.g., using lasers) and to receive a fixed wavelength. For example consider four channels 1, 2, 3 and 4. If the transmitting end is sending via channel 1, the receiving end must tune into the channel 1 wavelength as well to receive the data signal. When the transmitting end switches to channel 2, the receiving end must follow as well. Existing systems have as many as 2-128 signal channels.
In WDM, add/drop filters are needed to direct traffic in Long-Haul or Metropolitan networks. Current drop filter implementations lack flexibility. Some implementations have fixed wavelength drop filters. In these filters, each add/drop filter is fixed, meaning it is configured to extract and transmit only a specific wavelength within the optical fiber. This limits the flexibility for bandwidth allocation that WDM can provide.
In other filters where switching is allowed, the switching is often done in a non-hitless manner, meaning data is lost or interrupted during switching. Achieving hitless (non-blocking) wavelength switching is a challenge in drop filter design. In many critical applications the loss of data signal or interruption of service during wavelength switching is unacceptable. In these applications, the ability to hitlessly select a new wavelength without interruption of data flow is a requirement. However, many existing implementations of prior art tunable add/drop filters do not have this hitless property.
Typical examples of tunable optical filters include Fabry-Perot based tunable filters (xe2x80x9cFabry-Perot Tunable Filters Improve Optical Channel Analyzer Performancexe2x80x9d, Calvin Miller, Lawrence Pelz, Micron Optic Corp. and Siemens Corp.), ring resonator tunable filters (xe2x80x9cMicro-ring Resonator Channel Dropping Filtersxe2x80x9d, B. E Little, S. T Shu, H. A. Haus, J. Foresi, J.-P Laine, Journal of Lightwave Technology, vol 15. No 6, 1997), Fiber Bragg grating (FBG) tunable filters (xe2x80x9cBragg grating Fast tunable filter for wavelength division multiplexingxe2x80x9d, A locco, H. G Limberger, R. P Salathe, L. A. Everall, K. E. Chisholm, J. A. R Williams, I. Bennion), thin film tunable filters (www.santec.com), Acousto-Optic tunable filter (xe2x80x9cTi:LiNbO3 Acousto-optic tunable filter (AOTF)xe2x80x9d, T. Nakazawa, S. Taniguchi, M. Seino, Fujitsu, Sci. Tech. J, 35, 1, pp 107-112, 1999), Mach-Zehnder interferometers and electro-optic tunable filters. A review of these type of tunable optical filters is presented in xe2x80x9cTunable Optical Filters for Dense WDM Networksxe2x80x9d, D. Sadot, E. Boimovich, IEEE communication magazine, December 1998, page 50-55.
Fabry-Perot (FP) and ring resonator (RR) filters are based on the same principle: light bounces back and forth between two high reflectivity mirrors or circulate multiple times in the ring. Tunability is achieved by changing the optical path between the mirrors (or in the ring). By tuning from one wavelength to another, all wavelengths in-between are being swept during tuning yielding a blocking tuning. Fiber Bragg gratings use a periodic perturbation of the refractive index of a material to selectively reflect a particular wavelength: tunability is achieved by changing the period of the perturbation by applying mechanical or thermal stress. This tuning mechanism is blocking as well.
Tunable thin film filters are made by deposition of multiple layers of varying thickness and index of refraction. Tunability is achieved by spatially varying the layer thickness. Acousto-optic filters rely on the modulation of the index of refraction by the interaction of a acoustical wave launched in the material with a transducer. Tunability is achieved by varying the frequency of the acoustical wave. Although such a tuning mechanism is non-blocking (hitless), these filter are relatively broad-band ( greater than 1 nm) and difficult to fabricate.
The present invention provides a multi-channel tunable filter and methods for making (or recording) such a filter. In one embodiment, the tunable filter comprises a bank of gratings imprinted into a holographic substrate material, such as Lithium Niobate. Each grating reflects light at a specific wavelength, allowing light waves of all wavelengths except one to pass. In another embodiment, the tunable filter comprises a bank of thin film filters. For thin films, each grating reflects all wavelengths except a specific one. This allows light wave of only one wavelength to pass. An optical read-head comprising a pair of lenses (e.g. one dual fiber collimator and a single fiber collimator or two dual fiber collimators or two single lens collimators on each side) is configured to collimate the light which is then sent through an appropriate grating. The light reflected (a single wavelength for the holographic gratings and all wavelengths minus one for the thin film) is captured by the collimator positioned appropriately on the same side as the input collimator, the remainder of the channels (all minus one for the holographic gratings and a single wavelength for the thin film) are captured by a collimator positioned on the opposite side of the filter. The collimator receiving the filtered wavelength is called the drop collimator. Conversely, the drop collimator can also configured to add a wavelength instead of dropping a wavelength. Thus the tunable filter of the invention can add or drop one of the wavelengths from an optical fiber carrying multiple wavelengths.
In one embodiment of the present invention, the optical head is configured to move in a manner that avoids passing light waves through other gratings when a new grating is selected. This enables the tunable filter to achieve non-blocking (or hitless) architecture. If a different wavelength is desired to be added or extracted from the same fiber, the optical head moves to the appropriate grating in this hitless manner. In an alternate embodiment, the tunable add/drop filter of the present invention can be implemented in a blocking implementation.
In one embodiment, the gratings are made (or recorded) by the interference of two light beams. We call this method holographic. The angle between both beams and other parameters determine the period of the grating and hence the wavelength that has to be selected. Since the filter contains several gratings with different periods, the holographic material has to be illuminated by multiple pairs of beams at different positions over the material.
To record the gratings simultaneously two different recording techniques can be used:
1) A first plane wave reflects off a first mirror stack and a second plane wave reflects off a second mirror stack. The mirror stack is made of individual mirrors that are piled together with the required angle difference. The relative angle between each successive mirror is determined by the required channel spacing.
2) In another embodiment, the gratings are recorded by a phase masking method.